The present invention relates to a masking member used to protect the inside and circumference of a hole such as a water outlet hole, a cable piercing hole and the like from a surface treatment such as coating, plating, phosphatizing, vacuum evaporation and the like. More particularly, the present invention relates to a new masking member consisting of a vessel form inserting part made up of a bottom and a truncated conical wall portion, said wall portion extending upwardly from the perimeter of said bottom, and an outwardly extending flange formed around the wall portion of said inserting part, said masking member be made of a thermoplastic foam. In a case where a surface treatment is effected on the surface of an article, said surface of said article often has hole(s) whose inside must be protected from said surface treatment. Said hole(s) may be a water outlet hole, a cable piercing hole and the like, and a grummet, a plug, a bolt and the like may be inserted into said hole(s).